Organic light-emitting diodes have undergone a very rapid development in recent years. In particular, efficiencies of more than 100 lumen/W were successfully obtained for white-emitting light-emitting diodes. As also the lifetime of these systems has grown very quickly and, in the meantime, values of 10,000 hours for some material systems have even been significantly exceeded; organic light-emitting diodes also appear interesting for applications in lighting systems. The essential advantages of the organic light-emitting diodes are the high efficiency, the possibility of realizing very thin, large area, surface lighting units.
The conventional structural arrangement of organic light emitting diodes comprises a transparent substrate, in most cases glass, which is coated with a transparent anode that is frequently formed from indium tin oxide (ITO). Active organic layers and a metallic cathode are subsequently deposited over the anode. If some volts are applied between the metallic cathode and the transparent anode, the light-emitting diode emits the light through the substrate (bottom emission). Another variation is the top emitting OLED, in which the top electrode (anode or cathode) is transparent. Top emitting OLEDs can be constructed under a variety of substrates, for example metal substrates, Si-wafers, display backplanes, printed circuit boards (PCB), or even transparent substrates coated with a reflective layer, which reflective layer can be the bottom electrode itself. Exemplary documents concerning OLEDs are U.S. Pat. No. 4,539,507, WO90/13148, US20040062949, US20040251816, and US2007051946.
OLEDs on metal substrate have also been proposed. These OLEDs are so called top-emitting, because the light is transmitted trough the top electrode and not trough the substrate.
Several different approaches have been formulated to create large area, homogeneous OLED lighting devices. The main problem is the current distribution across the surface. Usually the electrodes of the OLED are also the current supply layers (electrical bus), or an extra conductive layer is deposited over the electrode, which has to be thin if transparency is required. For large areas, the resistance of these thin conductive layers is too high, since the current required for a large area OLED is very high. The resistance is the cause of the strong inhomogeneity in light emission and the device degradation.
Some approaches for homogeneity correction is the use of additional current supply lines, for example a metal grid deposited in connection to both electrodes. This grid lowers the resistance, but also slightly decreases the active area since it is not transparent. The metal grid also has a limited thickness, limiting again the maximum current that can be supplied to the OLED without loosing homogeneity. For higher currents, a denser grid is necessary, which decreases the efficiency of the whole device. Sometimes a grid is also not desired, because it again depreciates the homogeneity.
Another source of efficiency loss is the power lost by the electrical supply connections. The electrical connections are formed on the substrate and provide an electrical connection outside of the encapsulating region to the electrode itself, which is inside the encapsulated region. The distance of this thin film electrical connection is several cm, or several mm, in the best cases. The film can be larger, but not much thicker, because thicker layers would be not practical to deposit and would cause problem on the encapsulation.
The state of art electrical connections cause not only a big loss in power transmission to the OLED; their resistance and the large current cause considerable heating of the substrate, which does further depreciate the operational lifetime of the device.